Preheat the oven to 350°F and line two baking sheets with parchment paper. In the bowl of a handheld mixer or standing mixer, beat the butter, brown sugar, and granulated sugar together until light and fluffy (around 1-2 minutes). Add in the egg and vanilla extract. Add in the flour, salt, baking soda, and cocoa powder and mix until just combined. Stir in the chocolate chips and marshmallows.
Roll the dough into 2 tablespoon sized balls and place on the baking sheet around 3 inches apart. Press a few extra chocolate chips and marshmallows into the tops of the dough. Bake one tray at a time for 8-10 minutes or until the edges are set. Let the cookies cool for 10 minutes on the baking sheet, then transfer to a wire rack to cool completely.